Comparing SCSI and ATA
During the development of both SCSI and ATA devices, the design criteria
was different for both.
ATA was primarily designed with cost in mind, for low duty cycle
environments such as desktop PCs and laptops.
SCSI was developed for high end systems requiring higher performance
and higher duty cycles such as 24/7 operations.
From this we can see that ATA drives used the host CPU to help reduce cost,
whereas the SCSI devices used a hardware controller which in turn minimizes
the host CPU load.